1. Field of the Invention
The present invention relates to a light source and, particularly, to a high intensity light source for use in a projection system.
The invention has particular application to light sources for use with spatial light modulator devices.
A spatial light modulator is an optical device which is controllable so as to modulate an incident light beam. Colour spatial light modulators are known in which beams of different colours are reflected from different spatial light modulator devices, each driven in accordance with a different video signal. The coloured modulated beams are then combined to form a single projected colour display.
2. Description of the Prior Art
One known type of light modulators is an active matrix device, comprising a matrix of individually addressed pixels in the form of light valves or modulators. A system having an array of liquid crystal light modulators is described in EP-A-0401912 in which light is variably transmittable through each element of the array which in turn modulates the amplitude of the light passing through that element.
A tiltable mirror device is disclosed in U.S. Pat. No. 4,856,863, which shows devices having miniature mirrored elements, wherein each element includes electrodes and is arranged to be electrostatically deflectable between two positions, the extent of deflection being controllable by the extent of the applied electrostatic potential. Such devices may also be operated in a binary mode, in which each mirrored element is arranged to switch between two discrete deflection states, so as to reflect incident light into either a first position or a second position, so as to represent light or dark in the final output beam.
Using a tiltable mirror device, as each mirrored element is individually addressable, a two dimensional image can be reproduced by exposing the array to an incident light beam, modulating the incident beam by controlling the individual mirror devices from a video signal and collating the beam reflected in a particular direction. The small size of the mirrored elements, together with their very fast switching times, allows the elements to be operated at video rates, facilitating the display of a real time video image.
The incident beam does not scan the array, in the way in which an electron beam scans in a cathode ray tube, but is arranged to illuminate the entire device. Thus, given that a high intensity output is desirable in a projection system, it is desirable to illuminate the device with a high intensity beam. A system of this type is described in international application WO91/15843, assigned to the present assignee.
Not only must a high intensity beam be supplied to the array of devices, but the beam must be substantially uniform and be generated by a compact light generating means, in order that the overall dimension of the projection device be manageable.
A compact high intensity light source is manufactured by ILC Technology Inc. of California U.S.A. consisting of a compact xenon arc lamp, arranged to operate with an input power supply of one killowatt to produce a two inch diameter beam.
Much of the radiated energy produced in such a device cannot be used, either because it cannot be focused into the beam or because it does not lie within the visible spectrum. In order to increase the power output, it would be possible to increase the power input. There is, however, a limit to the amount of power which may be supplied to the device, due to electrode wear, overheating and general saftey constraints.
It is an object of the present invention to provide an improved light source. It is a further object of the present invention to provide an improved light source having improved light efficiency. It is a further object of the present invention to provide an improved light source with improved heat dissipation. Furthermore, it is an object of the present invention to provide an improved light source arranged to operate with an increased power input.
Another problem with known light sources is that the beam tends not to be uniform across its diameter. In particular, a central hole is often present due to the presence of the arc generating electrodes which obscure light as it is reflected back from an associated reflector.
When operating over larger distances, beam divergence ensures that the central hole is filled in and, when the light source is used as a searchlamp for example, no noticeable hole is present and the distribution of light across the beam width appears substantially Gausian. However, at short distances, such as those present in projection systems, the presence of the hole is noticeable and the distribution of light across the beam diameter is noticably non-isotropic.